Exporting and Importing the list of installed packages in one file for backup propouses

After installing ubuntu, experienced users, installs series of packages for different tasks. If you´re a developer probably you will install Build-Essentials, MySQL, PHP, and so on... If you have (or want) to format your computer, and install ubuntu again, you probably will need to find this packages and install them again. This work would be innecesary if there were an option to "export your installed packages configuration" in one file. This file could be in your Mail or in a backup drive. So if you have to re-install ubuntu, after that, you will only need to load this file to get your ubuntu ready for work. (The file, will only contain the installed packages and them dependencies). This could be a very usefull option, and easy to implement. Maybe for making "personal distributions of ubuntu"